[dtbook-to-zedai] New rules for `a` elements

 * if the `a` element has a @smilref but no @href, it is considered a mere
 anchor an consequently converted as a `span` (isntead of a `ref`)
 * @id and other global attributes are converted through

Comes with new unit tests.
Merge pull request #4 from daisy-consortium/issue-272

[dtbook-to-zedai] New rules for `a` elements